Immunosuppressive Therapy After Cardiac Transplantation (Treatment Strategies in Cardiac Transplantation) Part 2

Enteric-coated Mycophenolate sodium

Enteric-coated Mycophenolate sodiumfortic (EC-MPS) is an enterie formulation of mycophenolate sodium (a prodrug of MPA). MPA reversible inhibits the inosine monophosphate dehydrogenase and the pathway of guanosine nucleotide synthesis which affects B and T lymphocytes whereas other cell types can utilize salvage pathyways for purine synthesis. The coating of mycophenolate sodium should reduce the gastrointestinal side effects [59]. In renal transplant recipients a dosage of 720 mg EC-MPS twice a day was therapeutically equivalent to MMF 1000 mg twice a day with comparable safety profile [60].

Dosage recommendation in HTx recipients is 720mg twice a day either orally or intravenously. Optimal measurement of EC-MPS plasma concentration due to its delay in reaching maximal blood concentrations compared to MMF, is yet not clarify (C0, C2, C4, C6).

Proliferation signal inhibitors

Proliferation signal Inhibitors (PSI) (also named mammalian target of rapamycin (mTOR) inhibitors) include two important drugs currently available for organ transplantation: Rapamycin (Rapa) or Sirolimus (SRL) and Everolimus (EvE). Four decades ago Rapa was extracted and its antifungal effects reported [61]. Intensive research resulted in the discovery of the target of rapamycin named mTOR. mTOR is a serine-threonine kinase which is a transducer of information from growth factors and energy sensors within the cell. Both drugs form a complex with the intracellular binding protein FKBP-12, (similar to FK 506) but contrarily to TAC the PSIs inhibit the activity of mTOR. This leads to an arrest of a cell cycle in the mid-to-late G1 phase [61, 62]. While FK 506 is suppressing lymphokine production and blocking activation of T-cells, PSIs inhibit cells proliferation by impairing their response to growth-promoting lymphokines [63, 64]. They are also used in other areas of medicine like oncology or interventional cardiology (drug eluting stents).


Rapamycin

Rapamycin (Rapa) is a macrocyclic lactone with antifungal, antibiotic and IS properties. It was discovered in 1965, extracted out of soil taken from Rapa Nui in New Zeeland [65]. Its IS effects were discovered in the 1990s [61]. During the approval studies for Rapa the anti-tumor effects of Rapa and its analogues like EvE were found introducing them in oncology and for the prevention of restenosis after percutaneous coronary angioplasty.

Rapa has structural similarities to FK 506 binding protein but it forms complex with FKBP12 which results in an inhibitor of the mTOR [66]. This leads to suppression of T and B cells and decreases the population of dentritic cells who present antigen to T cells during activation [67].

The bioavailability of Rapa is 20% and decreases with food rich in fats (see 3.5.2); 92% of Rapa binds to albumin, is metabolism extensively in intestinal wall via p-glycoprotein and in the liver by CYP3A4. Seven major metabolites are known but 90% of the IS activity is done by Rapa; close to 90% is eliminated by the liver only 2% by the kidney. In contrast to EvE half-life time of Rapa is about 62 hours ± 16 hours allowing one single daily dose.

A loading dose for Rapa on the first post-transplantation day is recommended; in renal transplantation the loading dose should be 3 times the estimated maintenance dose (normally 2mg), in HTx recipients 15mg are given followed by a maintenance dose of 5mg and further guided by trough levels. The total dosage must no exceed 40mg per day; if a higher dose is needed it should be divided over a period of 2 days. In children with a body weight below 40 kg initially a loading dose of 3mg/ m2 and a maintenance dose of 1mg/m2 daily is recommended. If CNI therapy is reduced, Rapa dosage should be increased according to the targeted trough levels. In patient with severe hepatic impairment Rapa dosage should be reduced.

Routine clinical measurement is done with chromatographic methods. Major side effects of Rapa are swelling in different tissues, prolonging healing of wounds, increasing cholesterol and triglyceride levels, proteinuria as well as blood pressure. Rapa induced interstitial lung disease like pneumonitis have been observed [68-70]. When combined with CNIs, CNIs dosage reduction is necessary otherwise worsening renal function will develop. Rapa recommended blood trough levels in combination with CNIs is between 4 to 12 ng/ ml, without CsA a four times higher Rapa dosage might be needed (CsA/CNIs suppress the metebolizion of Rapa), the recommended blood trough levels is increased between 12 to 20 Ng/ ml depending on the time after transplantation. This is also the reason why Rapa intake when combined with CNIs is recommended four hours after CNI administration. Otherwise Rapa enhance the toxic effect of CNIs with an increased risk of CNI induced hemolytic uremic syndrome, thrombotic thrombocytopenic purpura and thrombotic microangiogiopathy. Drugs inducing CYP3A4 (Rifampicin) will decrease, strong inhibitors (Macrolides, Ketoconazole, Itraconazole) will increase Rapa blood levels. Similarly to CNIs grapefruit juice increases plasma concentration of Rapa. According to the last ISHLT report Rapa is currently used up to 20 % of HTx recipients [12].

Everolimus

Everolimus (EvE) is an analogue of Rapa and differs only by one extra hydroxyethyl group at position 40; still this leads to some differences. EvE blocks growth factor-mediated proliferation of cells including vascular smooth muscle cell through a CA2+ independent signal [71]. Following oral intake EvE is rapidly absorbed and reaches its maximal blood concentrations after 1 to 2 hours. The oral bioavailability is approximately 30% [72-75] and it is altered by food; a high-fat meal is slowing down the absorption of EvE. It is recommended that EvE is taken constantly either with or without food. EvE undergoes major metabolism with none of the metabolites reaching significantly IS activity. Its half-life time is 28 hours and compared to Rapa (62 hours) much shorter. Initial dose may be 0.75 mg twice a day, no loading dose is necessary.

EvE has a more rapid time to steady state compared to Rapa (4 versus 6 days). EvE binds to plasma proteins about 75% to 80% and is mainly eliminated in the liver, only 5% are extracted across the kidney. In patient with severe hepatic impairment EvE dosage should be reduced. PSIs and CNIs are metabolised by cytochrome P4503A4 (CYP3A4) isoenzyme leading to reduced clearance of EvE when CNI is given. Pre-clinical research reported of no nephrotoxicity of EvE [76] but when it was first clinical used combined with full dose CsA it showed worsening renal function [77, 78]. For that reason FDA approval was refused, but the European Medicine Agency (EMEA) approved EvE for further studies. In a prospective multicentre study the possibility of dose reduction of CsA combined with EvE resulted in stable renal function without loss of efficacy [79]. Further trials confirmed this [41, 42, 80]. Besides this interaction drugs who strong induce CYP3A4 will decrease, strong inhibitors will increase EvE blood levels. Reported EvE blood trough levels are within 3 to 8 ng/ ml. Drug monitoring is done by HPLC coupled with mass spectrometry and an immunoassay is being developed. EvE showed to have antiproliferative effects delaying the onset of cardiac transplant vasculopathy and reducing the rate of CMV infections [77, 81]; it is increasingly used, up to 2.6 % in HTx recipients in the years 2008 and 2009 [12]. Due to the favourable effects it may be used in children and is currently investigated (RAD 2313).

Immunosuppressive regimes

At the beginning little was know about interaction, side-effects and combination of IS drugs. Nowadays with many different IS drugs acting at different receptors and stages of the immune system more effective and less toxic regimes may be used.

It was revealed that the combination of different acting IS drugs with adjusted dosage enhance their effectiveness and reduce toxicity. To avoid nephrotoxic side effects of CNIs and to achieve a high IS, over 50% of the centres reporting to the ISHLT are using an induction therapy (20% using polyclonal antibodies, 30% use IL2 receptor antibodies) [12]. Conventionally for maintenance therapy patients are treated with a triple drug regimens, consisting of a CNI (CyC, TAC), antiproliferative agent (AzA, MMF) and corticosteroids. Shortly after the introduction of CsA in 1980 Griffith and colleagues used CsA in combination with low-dose steroids in HTx recipients, tapering steroids from 200mg per day to 15 mg per day similar to the regime used by Starzel in renal and liver transplant recipients [82-84]. Combining CsA, AzA and Cortocosteroides, commonly called triple-drug immunsupression, evolved and showed improved survival for short, medium and long term follow-up [85, 86]. It increased 1 years survival after HTx from 60% to 80% and became the standard regime not only in the US but also in European countries over the next 30 years [87, 88]. The triple-drug protocol, even if modified (many centres skipping corticosteroids after a certain time) is still used around the globe. Adding a forth drug to the regime has been reported but became not standard [19, 89].

Still due to the well know side effects of IS, associated with a significant morbidity, discussion about reducing IS will continue. Reduce IS therapy with a mono or dual drug regimes are investigated. Recently a retrospective study involving 150 patients within 28 days after HTx maintaining recipients only on monotherapy with TAC has been published [90]. One has to notice that in IS monotherapy compliance is paramount and could result in a disastrous outcome. The conviction of currently experts in the field of IS is, that today’s "standard" immunosuppression may be replaced by IS individualized for each patient on the basis of genomic profile, baseline risks for rejection and infection, and perhaps serial assessments of immune response after transplantation [91].

Immunosuppression for acute rejection

Different principles of IS treatment after organ transplantation have been established over time. After HTx numbers of rejection episodes and immune reactivity are highest within the first 3-6 months. Therefore one of the principles is to use the highest intensity of IS immediately after surgery and decrease it over the first year (Induction therapy (see 3.1), corticosteroid weaning (see 4.1.2); lowering blood concentrations of IS agents). The second principle is to rather admit more IS drugs with non-overlapping toxic side effects at a low dose rather than a higher and more toxic dose of a single drug. Therefore monitoring of the IS drug trough levels is of great interest; special caution must be paid to interaction of the drugs (lowering or increasing the blood levels) or i.e. diarrhea when orally taken. The goal is to avoid over-immunosuppression, which leads to infection and malignancy. This on the other hand may lead to late acute rejection episodes even if it is rare [92]. Corner stone of the treatment are corticosteroids, both oral or intravenous, ATG (see 3.1 Polyclonal Antibodies),

IL-2 receptor blockers (see 3.2 Interleukin 2 receptor antibodies) or murine monoclonal antibody (see 4.2). The type of treatment depends on clinical status of the recipients (if the rejection is hemodynamic compromising [reduced cardiac output, decreased pulmonary artery saturation, elevated wedge pressure, reduced cardiac index]) the histology degree and severity of the rejection. Moderate to severe rejection episodes need therapy: intravenous corticosteroids for three to five days, intensify oral maintenance IS therapy and eventually change to another protocol; if there are recurrent rejection episodes TAC or EvE may be considered. In patients with hemodynamic impairment additionally polyclonal or monoclonal antibodies or plasmapheresis should be kept in mind.

Corticosteroids

Corticosteroids inhibit the synthesis of cytokines, but the exact mechanism of action in solving acute rejection is not jet completely understood. Steroids suppress besides i.e. IL-6, interferon gamma, TNF, the production of IL-1 resulting in a diminished production of IL-2 by activated T cells. In animal models it was reported that steroids induce lymphocytolysis which was not proved in humans. Synthetic pharmaceutical drugs with corticosteroid-like effect are used in a variety of treatments. Prednisone is the most used synthetic steroid and is five times more potent compared to cortisol. Its bioavailability is 70% when orally taken and it is metabolised in the liver. Natural steroid hormones have a very short half-life time, synthetic steroids like prednisone have a half-life time of 1 hour. The side effects of long-term corticosteroids are commonly known; dosage reduction below the cushing threshold or even weaning them off are valid options. Nevertheless for acute rejection episodes intravenously high dose corticosteroid treatment is still necessary and effective. After solving the acute phase of the rejection episode orally corticosteroids should be introduced to treatment for at least some time or if already part of the maintenance IS protocol its dosage should be increased.

Corticodsteroid weaning

Over 85% of the centres reporting to the ISHLT are currently using corticosteroids within the first year after HTx, after 5 years about 50% are still using corticosteroids [12, 46]. The negative side effects of steroids are well known such as i.e. weight gain, glucose intolerance, dyslipidemia, osteoporosis, or cartaract. Most of the rejection episodes are within the first year after HTx and most of the steroids can be taken off over a course of a few months. The rationale for diminishing the overall use of corticosteroids is the availability of new IS agents acting more selective compared to synthetic corticosteroids. Numerous protocols were established, most of them use a high dosage of corticosteroids intra-operative (when starting reperfusion) and within the first days (as part of an induction therapy). When oral dosage is given different possibilities are available such as i.e. fixed dose of 15 mg per day or prednisolon 0.05 to 2mg/kg divided by one to four doses per day. After weeks of months the dose is reduced achieving a dose below the cushing threshold. Some study groups report to take off corticosteroids as early as 8 weeks after HTx [90] or over a course for several months following a simple weaning protocol guided by daily cortisol measurments to avoid onset of adrenal insufficiency (level > 8 ^g/ dl continue to wean, otherwise continue steroid therapy) (Baran DA. A prospective trial of steroid discontinuation in stable heart transplant patients as guided by serum cortisol measurement. International Society for Heart and Lung Transplantation 2009, Abstract 431). Other weaning protocols decrese the daily prednisone dosage by 1mg each month starting at month 6 post HTx [93].

The question why long-term use of corticosteroids is still that present may have several reasons i.e. avoiding adrenal insufficiency or other potential effects when treatment is stopped but also the ‘heritage’ of this therapy as steroids once were nearly the only immunosuppressant choice for transplant recipients.


Monoclonal muromonab CD3 antibody

Muromonab-CD3 (brand name: OKT3) is a monoclonal antibody against CD3 antigen resulting in an inhibition of T-cell function by down regulation of CD3 positive cells. It was the first monoclonal antibody to be approved for clinical use in humans. Similar to polyclonal antibodies its way of administration is only intravenously. Recommended dosage is 5 mg per day, in pediatric patients (< 30kg body weight) initial dosage may be lowered to 2.5mg per day. The human body will produce human anti-mice antibodies, as OKT3 is like a mice-antibody explaining the loss of effectivity if given repeatedly. Toxic side effects besides the well know from all IS agents (higher infection rate higher rate of lymphoproliferative disorders) have been reported: cytokine-mediated first-dose reaction, pulmonary edema, aseptic meningitis, haemolytic-uremic syndrome. The first-dose reaction may include fever, rigors, nausea, vomiting, and diarrhea which will decrease with repeated exposure. Nevertheless steroids, antihistamines and antipyretics should be given along with OKT3 to minimize these side effects. It takes about a week after ending the OKT3 treatment until the T cell function returns to normal.

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